The present invention provides a stereo modulation system using a very narrow bandwidth which occupies a very small portion of the radio frequency spectrum.
More particularly, the present invention provides a good quality single sideband stereo modulation system in which the modulation is performed at low power levels. The signal is then amplified and a single sideband for each channel is passed through a high Q bandpass filter and then transmitted.
The transmitted signals may be received and detected by conventional radio type detectors tuned to the center frequency of the single transmitted sidebands.
A brief history of Radio Broadcasting shows that we have two systems for public broadcasting of music and voice. These are AM and FM. We also have a single sideband suppressed carrier but it is unsuitable for music.
Applicant is proposing a new method for stereo broadcasting using a compatible single sideband suppressed carrier system having broadcast quality. This is called Frequency Aperture Modulation or, FAM.
The existing sideband now in use obtains its signal by phase cancelling of the carrier which must be reinjected at the receiver end. The phase of this reinjected carrier must be exactly that of the transmitted signal and must be highly stable for good voice intelligence. With Frequency Aperture Modulation, reception at the receiver does not require reinjection as the carrier is keyed by the audio, all harmonics included. In the existing single sideband suppressed carrier, all the harmonics are cancelled out, making it unsuitable for quality music broadcasting.
Applicant proposes that two of these FAM signals be placed back to back, one for each stereo channel. Each channel is 9 Khz wide, requiring a total bandwidth of approximately 18 Khz of the broadcast spectrum. The receiver would have two separate IF and audio channels for stereo reception.
To generate a FAM signal for one channel we first amplitude modulate an inaudible sub-carrier of 50 Khz with audio that ranges from 50 Hz to 9000 Hz. This signal then passes into a phase modulated amplifier and combines with a higher frequency of 500 Khz derived from a crystal oscillator. The result is a carrier and two sidebands, 500, 450 and 550 Khz. The lower side frequency (450 Khz) is then passed through a filter and the carrier and upper side frequency are eliminated. The 450 Khz signal is then multiplied to a much higher frequency. This procedure gives the greater frequency deviation that is required. We now have a very high frequency signal that is frequency modulated with 50 Khz and audio. This signal can now be hetrodyned with another carrier, the difference being a frequency that falls into the AM broadcast band. The resultant signal is then amplified and passed into a filter that is 9 Khz wide. The filter being only 9 Khz wide, filters out the 50 Khz sub-carrier and only permits RF components with the audio to pass. With no audio modulating the 50 Khz there is no energy getting through, therefore no carrier. Please note, that the hetrodyning signal places one side of the side frequency of the sub-carrier so that it falls in the 9 Khz filter. The 50 Khz is also frequency modulated and the center 31 should not fall into the filter, as some distortion will result. This is the same as tuning an AM receiver to an FM signal. Note, also the 9 Khz filter determines the frequency transmitted. The other channel is similarly generated.
The advantages of transmitting a FAM signal are the same as those of our present single sideband suppressed carrier. Many commercial communication companies and radio amateurs are using single sideband suppressed carrier. The efficiency is very high and energy is saved as the amount of electric power required is far less. The space required in the radio spectrum is one half that of AM and FM systems. Due to the filtering the chances of spurious radiation are reduced and interference of stations is at a minimum as the carrier is not transmitted. Distortion that occurs in AM and FM due to phase cancellation of signal caused by two different transmission paths is mostly eliminated. This will enhance overseas transmissions.
Two stations on the same frequency will not hetrodyne and will make tuning out one or the other easier.
Reception of a Frequency Aperture Modulated signal is compatible and can be received on conventional AM receivers now in use. If one wants stereo immediately, two separate receivers can be tuned, one to the upper and one to the lower frequencies. One receiver will act as channel one and the other as channel two. Most of the circuitry used in conventional receivers is applicable and greater improvement is possible with a new design having two IF strips and two audio strips, one for each channel. Due to the narrow bandwidth the reception noise figures is improved. Overseas broadcasting of stereo is improved. Overseas broadcasting of stereo on the higher frequencies is possible. Another advantage is that with no carrier transmitted there is a great reduction of `birdies` caused by hetrodyning within the receiver itself.
The fidelity of Frequency Aperture Modulation is excellent as all the audio harmonics are transmitted. This feature is not possible with the present sideband systems. Using FAM for stereo, the two channels together will make it possible to hear the high frequencies as the ear combines the output of the two loudspeakers. On the air tests will show to what extent this effect obtains.